Introduction Basic elements of Java

Transcription

1 Software and Programming I Introduction Basic elements of Java Roman Kontchakov / Carsten Fuhs Birkbeck, University of London

2 Module Information Time: Thursdays in the Spring term Lectures: MAL B04: pm 1 19 Torrington Pl, Room B17, A J: pm L Z: pm Labs: MAL 109 Optional tutorial hour: MAL 109, 5 6pm web: roman/sp1 moodle ( SP

3 Assessment In-Class Tests (weeks 5 & 11): 20% (10% & 10%) Programming Exercises 5% if you do not complete all exercises by week 10 then you will not be able to sit In-Class Test 2 and you will get 0 marks for the exercises Two-hour examination in summer 2019: 75% SP

4 Essential Textbook Cay Horstmann Java for Everyone 2nd edition John Wiley & Sons, st edition John Wiley & Sons, 2010 book also available online via BBK library, see roman/sp1/ the module draws on Chapters 1 9 and 12 the lab classes are based on exercises suggested in JFE SP

5 Python v Java Introduction to Programming was in Python. Why learn Java? Python particularly suitable for first steps Java widely used for large software systems concepts carry over from one programming language to another main difference: Java is statically typed goal: be(come) comfortable with more than one programming language NB: Java is not a version of JavaScript SP

6 Syllabus primitive data types and strings branching and loops arrays objects and classes methods and constructors instance and static variables and methods inheritance and polymorphism object-oriented design input/output basic data structures and algorithms SP

7 My First Program 1 /* HelloWorld.java 2 Purpose: printing a hello message on the screen 3 */ 4 public class HelloWorld { 5 // each program is a class (week 6) 6 // almost everything in Java is an object 7 public static void main(string[] args) { 8 System.out.println("Hello, " + args[0] + "!"); 9 } 10 } NB. watch out for semicolons they are compulsory NB. names and reserved words are case-sensitive SP

8 My First Program: Layout Style 2 1 /* HelloWorld.java 2 Purpose: printing a hello message on the screen 3 */ 4 public class HelloWorld 5 { // opening curly brackets on the new line 6 // each program is a class 7 public static void main(string[] args) 8 { 9 System.out.println("Hello, " + args[0] + "!"); 10 } 11 } // closing curly brackets directly below NB. different styles of curly bracket layout (indentation is irrelevant) SP

9 My First Program: No Style 1 /* HelloWorld.java Purpose: printing a hello message on the screen */ public class HelloWorld { public static void main(string[] args) { System.out. println("hello, " + args[0] + "!"); } } // all in a single line the Java compiler is happy, but... NB: println prints the argument and moves the cursor to the new line (ln comes from line ) print simply prints the argument SP

10 Java Development Environments Java Development Kit (JDK), Java }{{} SE 8 /11 Standard Edition BlueJ (a public project to make programming in Java easier) IntelliJ (extensible, free software with a proprietary commercial edition) Eclipse (multi-language and extensible, free and open source software) SP

11 Java Compilation and JRE source HelloWorld.java compiler javac bytecode HelloWorld.class }{{} JRE = JVM + (standard) classes Java Runtime Environment JDK = JRE + tools (compiler, etc.) java running program do not confuse the two! Virtual Machine (VM) SP

12 Java Bytecode: Example public static void main(java.lang.string[]); descriptor: ([Ljava/lang/String;)V Code: stack=4, locals=1, args_size=1 0: getstatic #2 // Field java/lang/system.out:ljava/io/printstream; 3: new #3 // class java/lang/stringbuilder 6: dup 7: invokespecial #4 // Method java/lang/stringbuilder."<init>":()v 10: ldc #5 // String "Hello," 12: invokevirtual #6 // Method java/lang/stringbuilder.append:(ljava/lang/ 15: aload_0 // first local variable (i.e., args) 16: iconst_0 // constant 0 17: aaload // load reference from array (i.e., args[0]) 18: invokevirtual #6 // Method java/lang/stringbuilder.append:(ljava/lang/ 21: ldc #7 // String "!" 23: invokevirtual #6 // Method java/lang/stringbuilder.append:(ljava/lang/ 26: invokevirtual #8 // Method java/lang/stringbuilder.tostring:()ljava/la 29: invokevirtual #9 // Method java/io/printstream.println:(ljava/lang/str 32: return SP

13 Compilation v Interpretation C source HelloWorld.c compiler machine code HelloWorld.exe executed on Python (and JavaScript) source HelloWorld.py interpreter interpreted by executed on hardware hardware SP

14 JDK: Editing source code can be edited in any text editor (e.g., Notepad, emacs,... ) MS Word caveat: by default, Word does not save in ASCII text format make sure to save the code before compiling! the file name must be the same as the name of the class (with the.java extension) (case sensitive!) SP

15 Compiling with JDK invoke the command-line compiler: javac <source>.java compiles <source> and all classes it depends on into Java bytecode files (<source>.java, etc.) for example: javac HelloWorld.java produces the file HelloWorld.class (provided there are no errors) make sure the compiler and JVM are in the command path (PATH) SP

16 Execution in JDK starting the Java Virtual Machine (JVM): java <source> the named class is loaded and execution is started (other classes are loaded as needed) only possible if the class has been compiled into Java bytecode How does the JVM know where to start the execution? SP

17 Coding in BlueJ BlueJ organises files into projects, stored in project-specific directories on disk do not forget to backup! types of BlueJ files: bluej.pkg: contains information about classes in the package (one per package) bluej.pkh: backup of the package file *.java: Java source code (text files, one per class) *.class: Java bytecode (binary, one per class) *.ctxt: BlueJ context file with extra information about the class (one per class) SP

18 Software is Free available on BBK s network JDK (allows one to compile and execute programs) BlueJ (preferred Java IDE) installing BlueJ for home use download JDK from download BlueJ from BlueJ tutorial SP

19 Comments 1 /* this is a block comment 2 comments provide additional information 3 that is not readily available in the code itself 4 comments are ignored by the Java compiler */ 5 public class HelloWorld { 6 // this is a single-line comment 7 public static void main(string[] args) { 8 System.out.println("Hello, " + args[0] + "!"); 9 } 10 } SP

20 Variables A variable is a storage location with a name cansperpack 6 When declaring a variable, you specify the type of its values (this is different from Python) and optionally its initial value int cansperpack = 6; char digit; initial value type variable name SP

21 Variable Names (aka Identifiers) variable names must start with a letter (or or $), the remaining characters must be letters,, $ or digits (but cannot be a reserved word) identifiers are case-sensitive by convention, variable names start with a lower-case letter Q: cansperpack cans per pack cans per pack cansperpack CaNsPeRpAcK digit 0digit digit0 int INT SP

22 Primitive Data Types int long short byte 32-bit two s complement integer 64-bit two s complement integer 16-bit two s complement integer 8-bit two s complement integer (-2,147,483,648 }{{} to 2,147,483,647 }{{} ) Integer.MIN VALUE Integer.MAX VALUE double double-precision 64-bit IEEE 754 floating point float boolean char single-precision 32-bit IEEE 754 floating point Boolean value (true or false) 16-bit Unicode character SP

23 Variable Assignment An assignment statement stores a new value in a variable, replacing the previously stored value (so, the previous value is lost) cansperpack = 8; variable name expression cansperpack 8 6 is lost Q: how do you swap the contents of two variables, a and b? int a; int b; a 4 b 5 (the answer is at the end) SP

24 Assignment v Equality The assignment operator = does not denote mathematical equality Q: what is the meaning of x = x + 1; Use == to compare numbers more in week 2 Pascal uses := for assignment and = for equality Software is getting slower more rapidly than hardware becomes faster (Niklaus Wirth, 1995) SP

25 Arithmetic Expressions Java uses the natural precedence of arithmetic operations: *, % }{{} remainder, / before +, - Q: what is the value of 2 * 6 / * 3? - + * / * (((2 * 6) / 4) + 5) - (2 * 3) answer: 2 SP

26 Expressions are Typed Java uses the natural precedence of arithmetic operations: *, %, / before +, - if in doubt, use brackets Q: what is the value of 2 * 6.0 / (5 + 3) - 2 * 3? double double / int * - double * int int 2 double answer: -4.5 SP

27 Integer Arithmetic Operations if one argument is double then the result is double if both arguments are int then the result is int Q: what is the value of 2 * 6 / (5 + 3) - 2 * 3? int / * - int * int 2 int NB: beware of the unintended integer division answer: -5 SP

28 Strings strings are sequences of characters: String name = "Harry"; }{{} literal the length method yields the number of characters in the string: int n = name.length(); the empty string "" is of length 0 SP

29 Strings Concatenation use the + operator to concatenate strings String name = "Harry"; String lastname = "Morgan"; String fullname = name + " " + lastname; NB: whenever one of the arguments of + is a string, the other argument is converted to a string (in Python a conversion function is needed) SP

30 Strings and Characters string positions are counted starting with 0 String name = "Harry"; H a r r y position char start = name.charat(0); char last = name.charat(name.length() - 1); // H // y do not confuse characters (of type char, e.g., H ) and strings containing a single character (e.g.,"h") SP

31 Substrings string that is made up of the characters starting at position i and containing all the characters up to, but not including, the position j: String greeting = "Hello!"; String sub = greeting.substring(0,2); all characters from the position i to the end of the string: String tail = greeting.substring(2); these are all examples of instance methods of the class String (week 6) SP

32 Methods A method is a named sequence of instructions method name public static int sq(int x) { java code (sequence of instructions) } parameter (type and name) type of return value Parameter values are supplied when a method is called The return value is the result that the method computes Method algorithm function (in Python) NB: until week 6, all methods will be public static SP

33 Some Remarks on Code Structure every method must be declared in a class that is, inside the curly brackets in class class-name {... } all code should occur in one of the methods until week 6: inside the curly brackets in public static return-type method-name(parameters) {... } one exception: initialisation blocks are not in the scope of SP1 the order of method declarations is not important (unlike in Python, where a function has to be declared before use) only System.out.println(...),.print,.printf, etc. actually print something on the screen (standard output) return does not print the value (and System.out.println(...) does not return any value to the program) SP

34 Example: y = x 2 as a Method 1 public class PrintSquares { 2 // compute xˆ2 3 public static int sq(int x) { 4 // x is a parameter variable 5 int y = x * x; // compute the value 6 return y; // return the value 7 } 8 public static void main(string[] args) { 9 System.out.println(7 + "ˆ2=" + sq(7)); 10 System.out.println(9 + "ˆ2=" + sq(9)); 11 } 12 } 7ˆ2=49 the output is: NB: void means that the method 9ˆ2=81 does not return any value SP

35 Swapping Values suppose there are two variables 1 int a, b; // the same as int a; int b; how do you swap the contents of a and b? a 4 b 5 2 int t = a; a 4 b 5 t 4 3 a = b; a 5 b 5 t 4 4 b = t; a 5 b 4 t 4 SP

36 Overview compiler, bytecode and JVM interpretation v compilation JDK and BlueJ variables: declaration and initialisation primitive data types arithmetic operations strings methods SP